Referring to FIG. 1, a typical hard disk drive 100 includes a disk drive base 102, at least one disk 104, 106 (such as a magnetic disk, magneto-optical disk, or optical disk), a spindle 108 for rotating the disk about a spindle axis of rotation 122, and a head stack assembly (HSA) 124. A printed circuit board assembly (not shown) is attached beneath the disk drive base 102 and includes electronics and firmware for controlling the rotation of the spindle 108, for controlling the position of the HSA 124, and for providing a data transfer channel between the disk drive 100 and its host.
The head stack assembly 124 typically includes an actuator 128, and a plurality of head gimbal assemblies (HGAs) 132. Each HGA 132 includes a head 134 for reading and writing data from and to a corresponding surface of disks 104, 106. In magnetic recording applications, the head typically includes an air bearing slider and a magnetic transducer that comprises a writer and a read element. The magnetic transducer's writer may be of a longitudinal or perpendicular design, and the read element of the magnetic transducer may be inductive or magnetoresistive. In optical and magneto-optical recording applications, the head may include a mirror and an objective lens for focusing laser light on to an adjacent disk surface.
During operation of the disk drive 100, the actuator 128 must rotate to position the heads 134 adjacent desired information tracks on corresponding surfaces of disks 104, 106. The actuator 128 includes a pivot bearing cartridge 136 to facilitate such rotational positioning. One or more actuator arms 130 extend from the actuator 128. An actuator coil 142 is supported by the actuator 128 opposite the actuator arms 130. The actuator coil 142 is configured to interact with a fixed magnet assembly 144 to form a voice coil motor. The printed circuit board assembly provides and controls an electrical current that passes through the actuator coil 142 and results in a torque being applied to the actuator 128. A latch/crash stop assembly 156 may limit excessive rotation of the actuator 128 in a given direction and/or when the disk dive 100 is not in use.
The spindle 108 typically includes a rotor including one or more rotor magnets, a rotating hub on which disks are mounted and clamped, a clamp 120 that is attached to the rotating hub (clamping one or more disks to rotate with the hub), and a stator. If more than one disk (e.g. disks 104, 106) is mounted on the hub, then the disks are typically separated by one or more spacer rings that are mounted on the hub between the disks. Various coils of the stator are selectively energized to form an electromagnetic field that pulls/pushes on the rotor magnet(s), thereby rotating the hub. Rotation of the spindle hub results in rotation of the clamp, spacer rings, and mounted disks.
Excessive imbalance of the disk mounting hub, disk clamp 120, disks 104, 106, and spacer rings (if any) of the spindle can cause undesirable disk drive vibrations and associated customer complaints. In extreme cases, such vibrations might even degrade the ability of the actuator to position the heads adjacent desired information tracks on the disk for reading and writing data. Therefore, it is advantageous to balance the hub, clamp, disk(s), and spacer rings (if any) of the spindle while or after they are assembled together.
In the example of FIG. 1, conventional balancing ring 112 is positioned at the top of disk clamp 120 of the spindle 108. The disk clamp 120 clamps the two disks 104 and 106 and associated spacer ring(s) to the hub of spindle 108 so that those disks and spacer rings rotate with the hub. The disk clamp 120 and conventional balancing ring 112 of the spindle 108 also rotate with the hub. The conventional balancing ring 112 includes a gap 114 along its circumference so that it is an open ring rather than a closed ring. The gap 114 constitutes a region of reduced mass that can be angularly positioned for balancing. Mechanical interference between a free position of the conventional balancing ring 112 and a corresponding groove in the disk clamp 120 creates a radial force that, through friction, resists incidental re-positioning of the conventional balancing ring 120 (while allowing deliberate re-positioning during balancing). Other conventional disk drive spindles have used selected discrete balancing masses inserted into one or more of a pattern of holes in the spindle hub, the holes being arranged at different angular positions, to accomplish balancing.
In the environment of modern disk drive manufacturing, thousands of disk drive spindles need to be balanced each day, and so tools (typically automated to some degree) have been developed to facilitate this. Such tools may be capable of adding, removing, or moving one or more masses on the hub, relative to the spindle axis of rotation 122, to counteract a net radial imbalance of the rotor (i.e. a net imbalance that would tend to dynamically translate the axis of rotation).
For example, such a balancing tool may measure an initial imbalance, and then select and affix a balancing ring of appropriate size and mass to the top of the disk clamp (e.g. balancing ring 112). Alternatively, for example, such a balancing tool may measure an initial imbalance, and then select and affix a discrete balancing mass through an opening in the disk clamp and then into one of a pattern of holes in the spindle hub, the holes being at different angular positions relative to the spindle hub.
In either case, the selection and positioning of an appropriate balancing mass or masses would depend upon the calibration of the associated imbalance measurement(s). Today, such calibration is typically accomplished by a manual process that may be burdensome and/or may not provide acceptable consistency. Thus, there is a need in the art for an improved process for practical calibration of the tools used to balance disk drive spindles in a high-volume manufacturing environment.